Gathered here are my ideas for a Warhammer 40,000 / Transformers crossover that currently doesn't exist. They draw heavily from the continuity and characterization of the recent Transformers animated series and movie, simply because these are most recent in my mind, but include references to other continuities as well, such as G1 and the Unicron trilogy. (Note that I never actually saw any of the Unicron trilogy, and all of my knowledge of it is drawn from the Transformers wiki. If you weren't worrying about the quality of my writing and characterization before, now is the time to start.)
May the God-Emperor and Primus forgive me, for none of you ever will.
Magos Technologis Isaac Sumdac
Magos Sumdac is a high-ranking tech-priest in the Cult Mechanicus, and unlike most tech-priests, is well-known and famous even outside that secretive organization. His discovery and translation of the Sumdac Fragment, a huge cache of Standard Template Construct data recovered from a pre-Imperial space probe, is what gave the planet Nebulos the high level of technology it now possesses. Nearly every piece of machinery on Nebulos, from civilian vehicles like cars and speeders, to military craft like the Seeker-class heavy fighter and the Transtector-class walker, to the nigh-omnipresent servitor drones that supplement the human labor and police forces, is based on STC data found in the Sumdac Fragment. Along with his "daughter," Sari, Magos Sumdac was one of the first humans on Nebulos to interact with the Autobots, and it was his idea to establish the Autobots' cover identity as experimental, transforming Praetorian battle-servitors.
In reality, of course, the "space probe" Sumdac discovered was the severed head of the Decepticon leader Megatron, and the STC data of the Sumdac Fragment was all discovered in the process of sifting through Megatron's memory banks. Sumdac knows perfectly well what the Autobots are, and he realizes the danger they pose both to him personally and to the planet as a whole; not only are Transformers sentient machines (thus making their very existence a form of techno-heresy), the AllSpark they carry will undoubtedly attract other Decepticons to Nebulos and risk bringing all-out war to the generally peaceful planet. Despite these risks, Sumdac remains a friend and ally of the Autobots, partially because he believes they deserve his trust, and partially because they're so very fascinating.
Spoiler: Sumdac's scientific "curiosity" extends further than anyone knows; in recent years, he has begun to incorporate more and more Cybertronian technology in his experimental creations, which has lead to no small number of these creations becoming semi-sentient and going berserk. His ultimate creation is Sari herself, a techno-organic cyborg incorporating both vat-grown human tissue and reactive Cybertronian machinery that grows and develops as Sari matures. Although Sumdac would vehemently deny it, his opponents' accusations that he has crossed the line into techno-heresy are well-founded.
Technographer Sari Sumdac
At fourteen years old, Sari is the youngest ordained member of the Adeptus Mechanicus on Nebulos, and possibly in the entire galaxy. Sari was born in a research colony on the planet Dredd, and was apparently the only survivor when the colony was destroyed by a hostile local lifeform. While recovering in a charity hospital on Nebulos, Sari was adopted as an official ward of the Adeptus Mechanicus by Magos Isaac Sumdac, effectively making her the Magos's "daughter." Sari, for her part, cannot remember any details of her life prior to entering the Cult Mechanicus, and a childhood spent among tech-priests, servitors, and heavy machinery is the only life she's ever known. Sari is well-versed in the rites and rituals of the Omnissiah, and has been officially granted the rank of Technographer, although she cannot receive the implants that mark her as a fully-inducted member of the Adeptus Mechanicus until she finishes growing. Sari is a friendly, cheerful, typical teenage girl for the most part, though a childhood spent growing up around machines and machine-priests has left her with little experience in dealing with other human beings.
Sari was one of the first humans on Nebulos to interact directly with the Autobots, as they came to her rescue after the Heavy Transtector she was piloting malfunctioned during a test run and threatened to destroy the city. Following them back to their damaged starship after the battle, Sari stumbled across the AllSpark, whose energies reacted with her identification card (for reasons she doesn't currently understand) to produce a strange, key-shaped object. This Key apparently served as a direct link to the energies of the AllSpark, granting it a miraculous ability to repair and induce transformations in seemingly any form of machinery -- an ability which would inexplicably intertwine Sari's and the Autobots' fate in the months to come.
Spoiler: Sari's pre-adoption history is a load of bunk; there was indeed a colony on Dredd that was destroyed by a local lifeform, but Sari wasn't in it. As stated above, Sari is a cyborg built by Magos Sumdac as a fusion of human biology and Cybertronian technology. Sari doesn't know this, and it's unlikely she'll take it well when she finds out.
Captain Carmen Fanzone
Carmen Fanzone is a captain in the Nebulos Enforcers and is in charge of the Central City Enforcer detachment. He hates machines, mostly because they have a tendency to malfunction around him. As such, he doesn't particularly like the Autobots, and feels somewhat uneasy whenever they're around; however, Magos Sumdac seems to have them under control, so he tolerates their presence. For now.
Officer Spike Witwicky
Spike Witwicky is a pilot in the Enforcers' T-squad, a Transtector-equipped elite unit deployed against heavily-armed or -armored criminals and recidivists. The son of a blue-collar widower, Spike decided he didn't want to spend the rest of his life toiling in one of the Powell family's manufactoria, and joined the Enforcers in an attempt to better his lot in life. After discovering that he was a decent Transtector pilot (no doubt thanks to his experience in operating heavy machinery), Spike found himself in the T-squad with four other pilots: Sergeant Chip Chase, Officer Carly Merriweather, Officer Bradley "Rad" White, and Officer Christopher "Kicker" Jones.
Spike, along with the rest of the T-squad, was present when the Autobots first appeared to bring down a rampaging Heavy Transtector. Unsure how to deal with the mysterious
robots "servitors" at first, Spike's repeated encounters with the Autobots, in which the Transformers and the T-squad often worked together to bring down some new threat, instilled in him a healthy respect for and a friendly familiarity with Optimus and his crew.
William Irving "Sparkplug" Witwicky
"Sparkplug" Witwicky is an assembly-line worker in a manufactorium owned by the Powell merchant family. Because of the ever-present danger of falling machine parts from overhead, Sparkplug always wears a hardhat to work, and has gotten in the habit of wearing it elsewhere as well.
Sparkplug has three sons by his late wife, who perished in an industrial accident. His oldest, Butch, takes after his father and is a worker at another Powell family manufactorium. His second son, Spike, is a Transtector pilot with the Enforcers. His youngest son, Buster, is a slightly effeminate young man who is studying to enter the Adeptus Ministorum. Sparkplug is immensely proud of his two older sons, and alternates between being proud of, confused by, and disappointed in his youngest; if nothing else, he's glad that Buster has proven at least some of his suspicions wrong by snagging himself a crazy-hot girlfriend. (What? Just because he's an old man doesn't mean he can't have an eye for... oh, never mind.) Sparkplug also has a brother named Ronald and a nephew named Sam, who also has a crazy-hot girlfriend.
Sparkplug isn't quite sure what to think of all this "Transformers" nonsense. The boys seem fond enough of them, but for his part, Sparkplug would be quite happy if he never saw one.
Spoiler: Yeah, right. Good luck with that one, Sparky.